Field of the Invention
The present invention relates to a capacitive transducer (Electromechanical Transducer) to be used as an ultrasound transducer.
Description of the Related Art
Hitherto, micromechanical members to be manufactured using micromachining technology may be processed on the order of micrometers, and various functional microelements are realized using such micromechanical members. An electromechanical transducer using such technology is being researched as an alternative to a transducer using a piezoelectric element. With such an electromechanical transducer, an acoustic wave (hereinafter sometimes represented by ultrasonic wave) may be transmitted or received using vibrations of a vibrating film, and in particular, excellent broadband characteristics in a liquid may be obtained with ease. Note that, the “acoustic wave” as used herein encompasses waves called a sonic wave, an ultrasonic wave, and a photoacoustic wave. For example, the “acoustic wave” encompasses a photoacoustic wave generated in a test object when the inside of the test object is irradiated with light (electromagnetic wave) such as visible light or infrared light.
Regarding the above-mentioned technology, there is known an electromechanical transducer exists that is formed of cells two-dimensionally arranged at regular intervals. Each of the cells includes a vibrating film and two electrodes opposed to each other with a gap interposed therebetween. Further, there is known an electromechanical transducer in which corrugated cells are two-dimensionally arranged or an electromechanical transducer in which means for blocking a signal is provided in an outer peripheral portion (see International Publication No. 2008/136198).
An output of an electromechanical transducer is determined depending on a sum of signals of time-varying interelectrode distances of a plurality of cells sharing a signal extraction electrode. Thus, the output is correlated with a sum of vibration velocities of a plurality of vibrating films. In the case of an electromechanical transducer having an infinite size, the vibration velocity of the vibrating film is uniform with regard to all the cells, and no unnecessary signal is generated. However, when cells that form an electromechanical transducer having a finite size are two-dimensionally arranged, a cell in an outer peripheral portion and a cell in a center portion of the electromechanical transducer have different boundary conditions. Therefore, when a plane wave enters the transducer, the vibration velocity of the vibrating film differs between the cell in the outer peripheral portion and the cell in the center portion. Such difference in vibration velocity of the vibrating films prevents uniformity of the vibration velocities of the vibrating films in the electromechanical transducer. Distribution of the cells with non-uniform vibration velocities of the vibrating films changes over time due to interaction between the cells.
The change over time is described with reference to FIGS. 5A to 5D. FIG. 5A is a graph showing a relationship between time and output. FIG. 5B, FIG. 5C, and FIG. 5D are sectional views illustrating vibration velocity distribution of vibrating films of an electromechanical transducer. Velocity of each of the cells in one element 1 in the electromechanical transducer is denoted by a reference numeral 2. First, when a plane wave enters the electromechanical transducer, due to difference in boundary condition, a section of vibration velocity distribution of the vibrating films is as illustrated in FIG. 5B. Next, due to interaction between the cells, it is seen that positions of cells that include the vibrating films having a high vibration velocity in FIG. 5B travel toward opposed sides, respectively, as illustrated in FIG. 5C. In this case, cells that include the vibrating films having positive and negative vibration velocities that are the same in magnitude exist in the electromechanical transducer, and thus, the output is small (see around t2 in FIG. 5A). Finally, the positions of cells having the high vibration velocity in the vibration velocity distribution that is seen to travel toward the opposed sides reach the opposed sides as illustrated in FIG. 5D, and thus, the absolute value of the sum of the vibration velocities of the vibrating films in the electromechanical transducer becomes large (see around t3 in FIG. 5A). When the absolute value of the sum of the vibration velocities of the vibrating films becomes large, an unnecessary signal is generated, which makes it difficult to obtain a high quality image.
In the technology disclosed in International Publication No. 2008/136198, in order to reduce non-uniformity in vibration velocity of the vibrating films in the electromechanical transducer, an electromechanical transducer having a signal blocking portion is used. However, when such a signal blocking portion is provided, a ratio of cells from which an output can be taken out in the electromechanical transducer is reduced, and transmitting or receiving sensitivity thereof is liable to be reduced.